Many types of fluidized beds are known in the art. For example, U.S. Pat. No. 4,322,296 discloses a method for waste water treatment in a vessel having a fixed perforated plate and a movable perforated plate. A particulate carrier having a specific gravity greater than water is employed as the filtering medium within the vessel.
As a further example, U.S. Pat. No. 5,453,183 discloses a fluidized bed biological filter system for fish tanks. This system utilizes glass beads having a specific gravity greater than 1.5 times that of water as particulate media.
In these filtration systems, contaminated water is passed through a negatively buoyant particulate medium. Thus, these systems only accomplish one type of filtration, i.e., filtration through a particulate carrier having a specific gravity greater than water.